On the composition of airborne particles influenced by emissions of the volcano Popocatépetl in Mexico

A 17-day field campaign was carried out in April-May 1999 to determine the contribution that gaseous volcanic emissions make to the composition of solid particles, particularly to the presence and quantity of sulfates. The experimental site was located in Tonantzintla (in the State of Puebla), only 30 km E from the volcano Popocatépetl, which has been in an active phase since the end of 1993. An analysis of the carbon monoxide (CO) and sulfur dioxide (SO₂) in the ambient air identified volcanic influence in 6 out of the 17 days sampled. Particles collected in an 8-stage cascade impactor were analyzed for inorganic ions (by liquid chromatography). A non-parametric test indicates a significant difference on the total particle mass and the sulfate fraction between days with and without volcanic influence. This difference was predominantly observed in the stages that collect the smaller particles. Winds at 500 mb (roughly corresponding to the summit of the volcano) indicate a westerly transport from the volcano to the experimental site, even though surface winds do not show a clear signal of such a transport. Back trajectories from the experimental site were calculated and clearly indicated that air parcels on the days independently identified as with volcanic influence had indeed passed over the volcano.